It is well known that a liquid fuel such as gasoline or light oil is used as a fuel to be combusted in an internal combustion engine. However, a gaseous fuel such as methane is also used as the fuel. JP-2000-240525A discloses a fuel injector for gaseous fuel. In this case, a mixture gas of the gaseous fuel injected by the fuel injector and an intake air flowing through an intake pipe is ignited and combusted in a combustion chamber.
However, the gaseous fuel is harder to mix with air than the liquid fuel.